The Reagan administration has received reliable intelligence reports that China suggested in June that it might provide Iran with sensitive nuclear technology after Peking signed a 30-year atomic power agreement with the United States, according to well-placed sources in the U.S. intelligence community.

These reports from Peking were shared in a closed briefing of the Senate Foreign Relations Committee earlier this month at the insistence of Sen. Jesse Helms (R-N.C.).

Along with other intelligence reports about Peking's continuing nuclear exports to Argentina and South Africa, the disclosure about Iran subsequently triggered an allegation Monday by Sen. Alan Cranston (D-Calif.) that the Reagan administration has "systematically withheld, suppressed and covered up information known virtually throughout the executive branch" about Chinese violations of nonproliferation guarantees.

Cranston's charges that the Chinese shared nuclear technology with what he termed five "nuclear outlaw" nations -- Brazil, Argentina, Pakistan, South Africa and Iran -- are the latest hitch in the U.S.-China nuclear agreement signed by President Reagan in April 1984. The pact would allow U.S. nuclear contractors to supply reactor components and other technology to Peking.

Because of widespread criticism, however, the agreement was not submitted to Congress until last July while the administration sought additional Chinese nonproliferation assurances. Unless rejected by Congress, the pact automatically takes effect in January.

After months of agitation from senators seeking access to current intelligence on China's nuclear exports, the issue came to a head on Oct. 8. The Central Intelligence Agency at that time refused to brief the Senate Foreign Relations panel about Chinese nuclear proliferation, and committee members reportedly clashed sharply at the hearing with Richard T. Kennedy, the State Department official in charge of nuclear cooperation with China.

The department yesterday "categorically denied" withholding information from the Senate but sources on Capitol Hill yesterday indicated that some senators were not persuaded.

One well-placed source said CIA sensitivity about sharing information with the Senate stemmed from detailed disclosure in a report Sept. 23 by columnists Jack Anderson and Dale Van Atta of the CIA's evidence that China helped Pakistan build a nuclear bomb.

"We have a very serious source problem in Peking," one intelligence official warned, adding that the administration would rather risk angering some senators than compromise intelligence capabilities that have allowed close monitoring of China's nuclear program.

The administration defended the nuclear agreement yesterday as Vice President Bush, just returned from a six-day trip to China, said, "We wouldn't enter into any agreement that would cause an increase in the proliferation of nuclear weapons."

Critics charge that the administration made the pact to score a foreign policy success for Reagan despite serious Pentagon, CIA and congressional concern about whether China has aided nuclear programs of nations expressing interest in developing such weapons.

Intelligence officials said yesterday that the CIA has a well-documented case that China helped design Pakistan's first nuclear weapons, made of nuclear components and reactor technology acquired covertly over a decade.

In a Senate speech Monday, Cranston said, "My information is that China has either engaged in serious nuclear trade negotiations with, or actually has continued a series of nuclear exports to, each and every one of these five 'nuclear outlaw' nations, subsequent to Chinese discussions with Reagan administration officials on the importance of curbing such troublesome exports.

"China has, in the recent past, engaged in the most egregious effort in history to export nuclear bombmaking know-how," the senator added, referring specifically to Peking's assistance to Pakistan.

Regarding Iran, Cranston said, "China has not shown, in its discussions of nuclear commerce" with Iranian parliament Speaker Ali Akbar Hashemi Rafsanjani ". . . anywhere near the type of prudence we would expect."

Rafsanjani met with senior Chinese officials in Peking in late June. One well-placed official said yesterday that some of the apprehension about Chinese assurances on nuclear issues stems from U.S. problems with Chinese assurances on other issues.

For instance, this official said China asserts that it does not sell conventional arms to Iran, which is at war with Iraq, although U.S. intelligence has ample evidence that it does.

Cranston's reference to wide circulation of the intelligence reports in the executive branch dealt with disclosure in the top secret National Intelligence Daily of the China-Iran discussions.

That occurred after the visit here this summer of Chinese President Li Xiannian, who joined Reagan in signing the nuclear-power pact. The daily intelligence report is circulated to about 200 administration officials.